Assessment of electrode coupling of tissue ablation

ABSTRACT

An electrode catheter and a method for assessing electrode-tissue contact and coupling are disclosed. An exemplary electrode catheter comprises an electrode adapted to apply electrical energy. A measurement circuit is adapted to measure impedance between the electrode and ground as the electrode approaches a target tissue. A processor determines a contact and coupling condition for the target tissue based at least in part on reactance of the impedance measured by the measurement circuit. In another exemplary embodiment, the electrode catheter determines the contact and coupling condition based at least in part on a phase angle of the impedance.

This application is a U.S. National Stage of PCT/US2006/061714, filed 6Dec. 2006, which claims the benefit of U.S. provisional application No.60/748,234, filed on 6 Dec. 2005, which is incorporate herein byreference.

This application is also related to international application Nos.PCT/US2006/046565, PCT/US2006/061716 PCT/US2006/061712,PCT/US2006/061710, PCT/US2006/061711, PCT/US2006/061713 andPCT/US2006/046816, being filed concurrently herewith (“internationalapplications”). The international applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The instant invention is directed toward an electrode catheter and amethod for using the electrode catheter for tissue ablation. Inparticular, the electrode catheter of the present invention may comprisea circuit to assess electrode-tissue contact and electrical coupling forapplying ablative energy (e.g., RF energy) to target tissue.

b. Background Art

It is well known that benefits may be gained by forming lesions intissue if the depth and location of the lesions being formed can becontrolled. In particular, it can be desirable to elevate tissuetemperature to around 50° C. until lesions are formed via coagulationnecrosis, which changes the electrical properties of the tissue. Forexample, lesions may be formed at specific locations in cardiac tissuevia coagulation necrosis to lessen or eliminate undesirable atrialfibrillations.

Several difficulties may be encountered, however, when attempting toform lesions at specific locations using some existing ablationelectrodes. One such difficulty encountered with existing ablationelectrodes is how to ensure adequate tissue contact and electricalcoupling. Electrode-tissue contact is not readily determined usingconventional techniques such as fluoroscopy. Instead, the physiciandetermines electrode-tissue contact based on his/her experience usingthe electrode catheter. Such experience only comes with time, and may bequickly lost if the physician does not use the electrode catheter on aregular basis. In addition, when forming lesions in a heart, the beatingof the heart further complicates matters, making it difficult todetermine and maintain sufficient contact pressure between the electrodeand the tissue for a sufficient length of time to form a desired lesion.If the contact between the electrode and the tissue cannot be properlymaintained, a quality lesion is unlikely to be formed. Similarly,information on electrical coupling between the electrode and the targettissue is not readily available a priori to determine how much ablativeenergy may be absorbed in the tissue during ablation. Instead, thephysician uses generalized pre-determined ablation parameters, such aspower and duration, based on his/her experience to perform ablationprocedures with the electrode catheter. Such experience may lead todeficiencies, inefficiencies and complications, such as inadequatelesion formation, premature high impedance shut-off, tissue charring,and thrombus formation.

BRIEF SUMMARY OF THE INVENTION

It is desirable to be able to assess electrode-tissue contact andelectrical coupling for electrode catheters used for tissue ablationprocedures. Although radio frequency (RF) ablative energy ispredominately resistive heating at typical operating frequencies ofabout 500 kHz, at lower frequencies there exist capacitances in thepatient's blood and tissue. The combined effects of resistance andcapacitance at the blood-tissue interface can be measured (e.g., asimpedance) to automatically assess different contact conditions betweenthe electrode and a target tissue.

An exemplary electrode catheter system may comprise an electrode adaptedto apply electric energy. A measurement circuit adapted to measureimpedance may be implemented between the electrode and ground as theelectrode approaches a target tissue. A processor or processing unitsmay be implemented to determine a contact condition for the targettissue based at least in part on reactance of the impedance measured bythe measurement circuit. In another embodiment, the contact conditionmay be based on the phase angle of the impedance.

An exemplary electrode catheter system may comprise an electrode adaptedto apply electric energy. A measurement circuit adapted to measureimpedance may be implemented between the electrode and ground as theelectrode approaches a target tissue. A processor or processing unitsmay be implemented to determine an electrical coupling condition for thetarget tissue based at least in part on reactance of the impedancemeasured by the measurement circuit. In another embodiment, theelectrical coupling condition may be based on the phase angle of theimpedance.

An exemplary method of assessing electrode-tissue contact for tissueablation may comprise: measuring impedance between an electrode andground as the electrode approaches a target tissue, separating areactance component from the measured impedance, and indicating acontact condition for the target tissue based at least in part on thereactance component.

An exemplary method of assessing electrode-tissue electrical couplingfor tissue ablation may comprise: measuring impedance between anelectrode and ground as the electrode approaches a target tissue,separating a reactance component from the measured impedance, andindicating electrical coupling condition for the target tissue based atleast in part on the reactance component.

Another exemplary method of assessing electrode-tissue contact fortissue ablation may comprise: directly measuring a phase angle betweenan electrode and ground as the electrode approaches a target tissue, andindicating a contact condition for the target tissue based at least inpart on the phase angle.

Another exemplary method of assessing electrode-tissue electricalcoupling for tissue ablation may comprise: directly measuring a phaseangle between an electrode and ground as the electrode approaches atarget tissue, and indicating electrical coupling condition for thetarget tissue based at least in part on the phase angle.

The contact condition may be conveyed to the user (e.g., a physician ortechnician), e.g., at a display device or other interface. The user maythen use the contact condition as feedback to properly position theelectrode catheter on the target tissue with the desired level ofcontact for the ablation procedure. For example, the user may increasecontact if the contact condition indicates insufficient contact. Or forexample, the user may reduce contact if the contact condition indicatestoo much contact.

The electrical coupling condition may be conveyed to the user (e.g., aphysician or technician), e.g., at a display device or other interface.The user may then use the electrical coupling condition as feedback toproperly position the electrode catheter on the target tissue with thedesired level of coupling for the ablation procedure. For example, theuser may increase coupling if the coupling condition indicatesinsufficient coupling. Or for example, the user may reduce coupling ifthe coupling condition indicates too much coupling.

It is also noted that in exemplary embodiments, a current source (oralternatively, a voltage source) may be used to administer theelectrical energy. This source can be the same source that is used forthe ablation procedure and is used to “ping” during positioning of theelectrode, or it can be a separately provided source. In any event, aconstant current source (or constant voltage source) may be used.Alternatively, a variable current source (or a variable voltage source),such as an ablation source operating in a mode that is adaptive totissue temperature. Furthermore, a plurality of the current sources (orvoltage sources) may be used. The plurality of current sources (orvoltage sources) may be operative either in a concurrent, sequential, ortemporally overlapping mode.

A number of additional aspects of the present invention exist. What maybe characterized as first through seventh aspects of the presentinvention each may be utilized to assess a coupling between an electrodeand tissue, which hereafter may be referred to as an “electrodecoupling.” This electrode coupling may be in the form of a mechanicalcoupling between the electrode and tissue, or stated another way acondition or state in which there is physical contact between theelectrode and tissue. Another embodiment has this electrode couplingbeing in the form of an electrical coupling between the electrode andtissue. Electrical coupling may be referred to as a condition or statewhen a sufficient amount of electrical energy is transferred from theelectrode to tissue. It should also be appreciated that there may be oneor more “degrees” of electrode coupling, and that one or more benchmarksassociated with a particular degree of electrode coupling may be tissuedependent.

A first aspect of the present invention is embodied by a medicalsystem/method for performing a medical procedure on tissue. A firstelectrode may be disposed in a certain position relative to tissue, anda first electrical signal may be sent to the first electrode. A phaseangle associated with the provision of this first electrical signal tothe first electrode is used to assess a coupling between the firstelectrode and the tissue (electrode coupling). More specifically, such aphase angle may be compared with at least one other phase angle value toassess the coupling between the electrode and the tissue.

Various refinements exist of the features noted in relation to the firstaspect of the present invention. Further features may also beincorporated in the first aspect of the present invention as well. Theserefinements and additional features may exist individually or in anycombination. Initially, the features discussed below in relation to thefifth aspect may be incorporated into this first aspect.

At least one phase angle benchmark value may be provided for the phaseangle comparison in accordance with the first aspect. In one embodiment,this phase angle benchmark value is stored within a data structure or isotherwise accessible by a phase angle comparator or the like. In oneembodiment, a phase angle benchmark value is associated with aninsufficient coupling condition. In another embodiment, a phase anglebenchmark value is associated with an elevated or excessive couplingcondition.

In one embodiment of the first aspect, one or more categories or rangesmay be provided for a phase angle comparison to assess the electrodecoupling. Any appropriate number of phase angle categories or ranges maybe used, and these phase angle categories or ranges may be determined orset in any appropriate manner (e.g., empirically). For instance: 1) afirst range may include those phase angles that are associated with aninsufficient coupling condition, and which may be utilized by a phaseangle comparator or the like to determine if a phase angle associatedwith the first electrical signal is within this first range; 2) a secondrange may include those phase angles that are associated with asufficient coupling condition, and which may be utilized by a phaseangle comparator or the like to determine if a phase angle associatedwith the first electrical signal is within this second range; and 3) athird range may include those phase angles that are associated with anelevated or excessive coupling condition, and which may be utilized by aphase angle comparator or the like to determine if a phase angleassociated with the first electrical signal is within this third range.Each of these first, second, and third ranges could be used individuallyto compare with a phase angle value associated with the first electricalsignal, or may be used in any appropriate combination with each other.It should be appreciated that what is “insufficient,” “sufficient,”“elevated/excessive” may be dependent upon the tissue being coupled withthe first electrode, as well as one or more other factors.

A phase angle associated with the first electrical signal at a certainpoint in time of a medical procedure may be determined in anyappropriate manner and for purposes of assessing the electrode couplingat this certain point in time in accordance with the first aspect. It ofcourse may be desirable to assess the electrode coupling on somepredetermined temporal basis or otherwise in accordance with somepredefined function (e.g., assess a phase angle associated with thefirst electrical signal every “x” seconds during at least part of amedical procedure). In one embodiment, the phase angle that isassociated with the first electrical signal is a phase angle between acurrent being provided to the first electrode and a voltage that existsbetween the first electrode and another electrode such as a returnelectrode.

A second aspect of the present invention is embodied by a medicalsystem/method for performing a medical procedure on tissue. A firstelectrode may be disposed in a certain position relative to tissue, anda first electrical signal may be sent to the first electrode. Areactance associated with the provision of this first electrical signalto the first electrode is used to assess a coupling between the firstelectrode and the tissue (electrode coupling). More specifically, such areactance may be compared with at least one other reactance value toassess the coupling between the electrode and the tissue.

Various refinements exist of the features noted in relation to thesecond aspect of the present invention. Further features may also beincorporated in the second aspect of the present invention as well.These refinements and additional features may exist individually or inany combination. Initially, the features discussed below in relation tothe fifth aspect may be incorporated into this second aspect.

At least reactance benchmark value may be provided for the reactancecomparison in accordance with the second aspect. In one embodiment, thisreactance benchmark value is stored within a data structure or isotherwise accessible by a reactance comparator or the like. In oneembodiment, a reactance benchmark value is associated with aninsufficient coupling condition. In another embodiment, a reactancebenchmark value is associated with an elevated or excessive couplingcondition.

In one embodiment of the second aspect, one or more categories or rangesmay be provided for a reactance comparison to assess the electrodecoupling. Any appropriate number of reactance categories or ranges maybe used, and these reactance categories or ranges may be determined orset in any appropriate manner (e.g., empirically). For instance: 1) afirst range may include those reactance values that are associated withan insufficient coupling condition, and which may be utilized by areactance comparator or the like to determine if a reactance associatedwith the first electrical signal is within this first range; 2) a secondrange may include those reactance values that are associated with asufficient coupling condition, and which may be utilized by a reactancecomparator or the like to determine if a reactance associated with thefirst electrical signal is within this second range; and 3) a thirdrange may include those reactance values that are associated with anelevated or excessive coupling condition, and which may be utilized by areactance comparator or the like to determine if a reactance associatedwith the first electrical signal is within this third range. Each ofthese first, second, and third ranges could be used individually tocompare with a reactance value associated with the first electricalsignal, or used in any appropriate combination with each other. Itshould be appreciated that what is “insufficient,” “sufficient,” or“elevated/excessive” may be dependent upon the tissue being coupled withthe first electrode, as well as one or more other factors.

A reactance associated with the first electrical signal at a certainpoint in time of a medical procedure may be determined in anyappropriate manner and for purposes of assessing the electrode couplingat this certain point in time in accordance with the second aspect. Itof course may be desirable to assess the electrode coupling on somepredetermined temporal basis or otherwise in accordance with somepredefined function (e.g., assess a reactance associated with the firstelectrical signal every “x” seconds during at least part of a medicalprocedure). In one embodiment, the reactance that is associated with thefirst electrical signal is a reactance associated with the electricalpath between the first electrode and another electrode such as a returnelectrode.

A third aspect of the present invention is embodied by a medicalsystem/method for performing a medical procedure on tissue. A firstelectrode may be disposed in a certain position relative to tissue, anda first electrical signal may be sent to the first electrode. What maybe characterized as an impedance components ratio associated with theprovision of this first electrical signal to the first electrode is usedto assess a coupling between the first electrode and the tissue(electrode coupling). This “impedance components ratio” is a ratio oftwo component values that define an impedance (e.g., resistance,reactance, impedance) that is associated with the provision of the firstelectrical signal. More specifically, such an impedance components ratiomay be compared with at least one other impedance components ratio valueto assess the coupling between the electrode and the tissue.

A fourth aspect of the present invention is embodied by a medicalsystem/method for performing a medical procedure on tissue. A firstelectrode may be disposed in a certain position relative to tissue, anda first electrical signal may be sent to the first electrode. Thedevelopment of an elevated or excessive coupling condition (e.g.,mechanical, electrical, or both) may be identified through anappropriate assessment.

Various refinements exist of the features noted in relation to thefourth aspect of the present invention. Further features may also beincorporated in the fourth aspect of the present invention as well.These refinements and additional features may exist individually or inany combination. Initially, the features discussed below in relation tothe fifth aspect may be incorporated into this fourth aspect.

One or more parameters may be monitored/assessed for purposes ofidentifying the existence of an elevated or excessive coupling conditionbetween the first electrode and tissue in the case of the fourth aspect,including without limitation impedance, phase angle (e.g., in accordancewith the first aspect), reactance (e.g., in accordance with the secondaspect), and target frequency (e.g., in accordance with the seventhaspect discussed below). A reactance (e.g., of a portion of anelectrical circuit that extends from the first electrode, through apatient's body, and to a return electrode) may be compared to at leastone reactance benchmark value to determine if an excessive couplingcondition exists. In one embodiment, an elevated or excessive couplingcondition is equated with a reactance that is less than a predeterminednegative reactance value. A phase angle (e.g., a phase angle between thecurrent at the first electrode, and the voltage between the firstelectrode and a return electrode) also may be compared to at least onephase angle benchmark value to determine if an elevated or excessivecoupling condition exists. In one embodiment, an elevated or excessivecoupling condition is equated with a phase angle that is less than apredetermined negative phase angle value.

That frequency for the first electrical signal at which the phase angleis at a certain, preset value (e.g., a phase angle between the currentat the first electrode, and the voltage between the first electrode anda return electrode) may be referred to as a “target frequency”, and thistarget frequency may be compared to at least one frequency benchmarkvalue to determine if an elevated or excessive coupling condition existsfor purposes of this fourth aspect. In one embodiment, an elevated orexcessive coupling condition is equated with having a target frequencythat is greater than a predetermined frequency value. That frequency forthe first electrical signal at which an inductance (e.g., of a portionof an electrical circuit that extends from the first electrode, througha patient's body, and to a return electrode) is at a certain, presetvalue may define a target frequency as well, and this target frequencymay be compared to at least one frequency benchmark value to determineif an elevated or excessive coupling condition exists. In oneembodiment, an elevated or excessive coupling condition is equated withhaving a target frequency that is greater than a predetermined frequencyvalue. Generally, an appropriate electrical parameter may be associatedwith a target frequency, and any appropriate value may be used for thiselectrical parameter for purposes of the target frequency. Frequenciesabove a target frequency may be associated with a certain condition,frequencies below a target frequency may be associated with a certaincondition, or both.

A fifth aspect of the present invention is embodied by a medicalsystem/method for performing a medical procedure on tissue. A firstelectrode may be disposed in a certain position relative to tissue, anda first electrical signal that provides a first current may be sent tothe first electrode. This first current is used to perform a firstmedical procedure (e.g., ablation of heart tissue). A coupling betweenthe first electrode and the tissue is also assessed using this firstcurrent.

Various refinements exist of the features noted in relation to the fifthaspect of the present invention. Further features may also beincorporated in the fifth aspect of the present invention as well. Theserefinements and additional features may exist individually or in anycombination. The coupling between the first electrode and the tissue inthe case of the fifth aspect may be assessed in any appropriateparameter. This assessment may be based upon impedance comparisons,phase angle comparisons (e.g., in accordance with the first aspect),reactance comparisons (e.g., in accordance with the second aspect), andtarget frequency comparisons (e.g., in accordance with the seventhaspect discussed below).

A second electrical signal that provides a second current may be sent tothe first electrode in the case of the fifth aspect. The couplingbetween the first electrode and the tissue may also be assessed usingthis second signal. Various characterizations may be made in relation tothe second electrical signal, and which apply individually or in anycombination: 1) the second current may be less than the first current;2) the first and second electrical signals may be at least generally ofthe same frequency; and 3) the first and second signals may be sentsequentially or other than simultaneously, for instance by switchingfrom one electrical power source to another electrical power source. Inthe latter regard, a switch may be disposed in one position tointerconnect the first electrode with a first electrical power source(e.g., an assessment power source), and a first electrode couplingassessment module may be used to assess electrode coupling. Disposingthis switch into another position may interconnect the first electrodewith a second electrical power source (e.g., an ablation power source),and a second electrode coupling assessment module may be used to assesselectrode coupling. These first and second electrode coupling assessmentmodules may be of a common configuration.

A sixth aspect of the present invention is embodied by a medicalsystem/method for performing a medical procedure on tissue. In oneembodiment, a first catheter having a first electrode is positionedwithin a first chamber of a patient's heart (e.g., the left atrium),along with a second catheter having a second electrode. In anotherembodiment, first and second electrode tips (e.g., associated withdifferent catheters; associated with a common catheter) are positionedwithin a first chamber of the heart. In each case, a first electricalsignal may be sent to the first electrode for performing a first medicalprocedure, and a coupling between the first electrode and tissue may beassessed using this first electrical signal.

Various refinements exist of the features noted in relation to the sixthaspect of the present invention. Further features may also beincorporated in the sixth aspect of the present invention as well. Theserefinements and additional features may exist individually or in anycombination. The coupling between the first electrode and the tissue inthe case of the sixth aspect may be assessed in any appropriateparameter. This assessment may be based upon impedance comparisons,phase angle comparisons (e.g., in accordance with the first aspect),reactance comparisons (e.g., in accordance with the second aspect), andtarget frequency comparisons (e.g., in accordance with the seventhaspect discussed below). In addition, the features discussed above inrelation to the fifth aspect may be incorporated into this sixth aspect.

A seventh aspect of the present invention is embodied by a medicalsystem/method for performing a medical procedure on tissue. A firstelectrode may be disposed in a certain position relative to tissue, anda first electrical signal may be sent to the first electrode. One ormore frequencies may be analyzed to identify a frequency where anelectrical parameter is of certain value (where “value” includes acertain range of values).

Various refinements exist of the features noted in relation to theseventh aspect of the present invention. Further features may also beincorporated in the seventh aspect of the present invention as well.These refinements and additional features may exist individually or inany combination. A target frequency may be where a frequency provides azero phase angle (e.g., a phase angle between a current being providedto the first electrode and a voltage that exists between the firstelectrode and another electrode such as a return electrode). A zerofrequency also may be where a frequency provides an inductance that iszero (e.g., an inductance of a portion of an electrical circuit thatextends from the first electrode, through a patient's body, and to areturn electrode). Any electrical parameter may be used for purposes ofthe target frequency, and this electrical parameter may be of anyappropriate value for purposes of a target frequency. In one embodiment,a target frequency is identified by sequentially providing a pluralityof electrical signals at different frequencies (e.g., using a frequencysweep), and determining which of these electrical signals generates anelectrical parameter of a requisite value. In another embodiment, anelectrical signal that includes a plurality of frequencies is sent tothe first electrode. Filters may be used to allow each of the variousfrequencies from this common electrical signal to be separately analyzedto determine if any of these frequencies generates an electricalparameter of a requisite value.

A target frequency may be used to assess the coupling between the firstelectrode and tissue in the case of the seventh aspect. In this regard,at least one frequency benchmark value may be provided for a frequencycomparison in accordance with the seventh aspect to assess electrodecoupling. In one embodiment, this frequency benchmark value is storedwithin a data structure or is otherwise accessible by a frequencycomparator or the like. In one embodiment, a frequency benchmark valueis associated with an insufficient coupling condition. In anotherembodiment, a frequency benchmark value is associated with an elevatedor excessive coupling condition.

In one embodiment of the seventh aspect, one or more categories orranges may be provided for a frequency comparison to assess electrodecoupling. Any appropriate number of frequency categories or ranges maybe used, and these frequency categories or ranges may be determined orset in any appropriate manner (e.g., empirically). For instance: 1) afirst range may include those frequencies that are associated with aninsufficient coupling condition, and which may be utilized by afrequency comparator or the like to determine if the target frequency iswithin this first range; 2) a second range may include those frequenciesthat are associated with a sufficient coupling condition, and which maybe utilized by a frequency comparator or the like to determine if thetarget frequency is within this second range; and 3) a third range mayinclude those frequencies that are associated with an elevated orexcessive coupling condition, and which may be utilized by a frequencycomparator or the like to determine if the target frequency is withinthis third range. Each of these first, second, and third ranges could beused individually to compare with a target frequency, or may be used inany appropriate combination with each other. It should be appreciatedthat what is “insufficient,” “sufficient,” or “elevated/excessive” maybe dependent upon the tissue being coupled with the first electrode, aswell as one or more other factors.

There are a number of features or the like that are applicable to eachof the first through the seventh aspects, and which will now besummarized. The first electrode may be of any appropriate size, shape,configuration, and/or type, and further may be used to execute any typeof medical procedure (e.g., ablation). In one embodiment, the firstelectrode is in the form of a catheter electrode.

The first electrical signal may be at any appropriate frequency in thecase of the first through the seventh aspects. In one embodiment andexcept in the case of the seventh aspect, only a single frequency isrequired for purposes of providing an electrode coupling assessment. Anyappropriate electrical power source or signal generator may be used toprovide the first electrical signal or any other electrical signal. Eachsuch electrical power source or signal generator may be continuallyinterconnected with the first electrode, or may be electricallyinterconnected as desired/required through operation of a switch or thelike.

A return electrode may be used in combination with the first electrodeto execute a medical procedure using the first electrode in the case ofthe first through the seventh aspects, and which also may be used for anelectrode coupling assessment. The following features relating to such areturn electrode may be used individually or in any appropriatecombination: 1) each of the first electrode and the return electrode maybe in the form of a catheter electrode, and each such catheter electrodemay be independently maneuverable; 2) the return electrode may utilize alarger surface area than the first electrode; and 3) each of the firstelectrode and return electrode may be disposable in a common chamber ofthe heart, such as the left atrium.

Any electrode coupling assessment used by the first through the seventhaspects may utilize at least one electrode coupling assessment module(e.g., an electrical circuit). Each such electrode coupling assessmentmodule may be incorporated in any appropriate manner and at anyappropriate location. For instance, an electrode coupling assessmentmodule may be incorporated into the catheter, may be in the form of astandalone unit, may be incorporated by an electrical power generator,may be incorporated by an electrophysiology mapping system, or may beincorporated by electrophysiology signal recording system.

Each of the first through the seventh aspects may be used to identifythe existence of an elevated or excessive coupling condition. Theability to identify the existence of such an elevated or excessivecoupling condition may be desirable for a number of reasons. Forinstance, it may be desirable to avoid an elevated or excessive couplingcondition (e.g., to reduce the potential of puncturing a tissue wall ormembrane). It also may be desirable to reach an elevated or excessivecoupling condition (e.g., to increase the potential of passing the firstelectrode through a tissue wall or membrane).

Any phase angle comparison used by the first through the seventh aspectsmay utilize a phase shift circuit to facilitate themeasurement/determination of a phase angle. For instance, the phase of acurrent signal being provided to the first electrode may be shifted anappropriate amount (e.g., by 90°). It also may be desirable tocompensate for a residual phase shift for purposes of any electrodecoupling assessment based upon a phase angle comparison. That is, aphase shift may be indicated to exist for an electrode couplingassessment, when there in fact should be no phase difference under thecurrent circumstances.

The result of any electrode coupling assessment used by the firstthrough seventh aspects may be output in any appropriate manner to oneor more locations. This output may be in the form of one or more ofvisual feedback, audible feedback, or physical feedback. For instance, abar graph or other display may be utilized to visually convey thecurrent degree of the electrode coupling. It may be desirable toscale/amplify the output of the electrode coupling assessment.

The foregoing and other aspects, features, details, utilities, andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary tissue ablationsystem which may be implemented to assess electrode-tissue contactduring a tissue ablation procedure for a patient.

FIG. 1 a is a detailed illustration of the patient's heart in FIG. 1,showing the electrode catheter after it has been moved into thepatient's heart.

FIG. 2 a illustrates exemplary levels of electrical contact or couplingbetween the electrode catheter and a target tissue.

FIG. 2 b illustrates exemplary levels of mechanical contact or couplingbetween the electrode catheter and a target tissue.

FIG. 3 is a high-level functional block diagram showing the exemplarytissue ablation system of FIG. 1 in more detail.

FIG. 4 is a model of the electrode catheter in contact with (or coupledto) target tissue.

FIG. 4 a is a simplified electrical circuit for the model shown in FIG.4.

FIG. 5 is an exemplary phase detection circuit which may be implementedin the tissue ablation system for assessing electrode-tissue contact orcoupling.

FIG. 6 is an exemplary block diagram showing phase angle measurement forcontact sensing and tissue sensing.

FIG. 7 is an exemplary block diagram showing phase angle measurementduring ablation with both ablation energy and a contact sensing signalapplied to the ablation electrode at the same time.

FIG. 8 is an exemplary block diagram showing phase angle measurementduring ablation with switching between a sensing signal and ablationpower.

FIG. 9 a illustrates one embodiment of a protocol that may be used toassess a coupling between an electrode and tissue based upon a phaseangle comparison.

FIG. 9 b illustrates one embodiment of a protocol that may be used toassess a coupling between an electrode and tissue based upon a reactancecomparison.

FIG. 9 c illustrates one embodiment of a protocol that may be used toassess a coupling between an electrode and tissue based upon animpedance components ratio comparison.

FIG. 10 illustrates a representative, schematic representation of anelectrical coupling between an electrode and tissue.

FIG. 11 a illustrates a schematic of one embodiment of an ablationsystem that uses two power sources operating at different frequencies,where only one of these power sources is interconnected with theablation electrode at any one time, and where one of these power sourcesis used for assessing a coupling between an electrode and tissue.

FIG. 11 b illustrates a schematic of one embodiment of an ablationsystem that uses two power sources operating at different frequencies,where both power sources are always interconnected with the ablationelectrode, and where one of these power sources is used for assessing acoupling between an electrode and tissue.

FIG. 11 c illustrates a schematic of one embodiment of an ablationsystem that uses two power sources operating at least generally at thesame frequency, where only one of these power sources is interconnectedwith the ablation electrode at any one time, and where each of thesepower sources may be used for assessing a coupling between an electrodeand tissue.

FIG. 12 a illustrates one embodiment of a system for assessing acoupling between an electrode and tissue.

FIG. 12 b illustrates one embodiment of a protocol that may be used toassess a coupling between an electrode and tissue based upon identifyinga baseline coupling condition.

FIG. 12 c illustrates one embodiment of a protocol that may be used toassess a coupling between an electrode and tissue based upon identifyinga target frequency.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of a tissue ablation system and methods of use toassess electrode-tissue contact and electrical coupling are depicted inthe figures. As described further below, the tissue ablation system ofthe present invention provides a number of advantages, including, forexample, the ability to apply a reasonable amount of ablative energy toa target tissue while mitigating electrode-tissue contact and couplingproblems. The invention also facilitates enhanced tissue contact andelectrical coupling in difficult environments (e.g., during lesionformation on a surface inside a beating heart).

FIG. 1 is a diagrammatic illustration of an exemplary electrode cathetersystem 10 which may be implemented to assess electrode-tissue contactduring a tissue ablation procedure for a patient 12. Catheter system 10may include an electrode catheter 14, which may be inserted into thepatient 12, e.g., for forming ablative lesions inside the patient'sheart 16. During an exemplary ablation procedure, a user (e.g., thepatient's physician or a technician) may insert the electrode catheter14 into one of the patient's blood vessels 18, e.g., through the leg (asshown in FIG. 1) or the patient's neck. The user, guided by a real-timefluoroscopy imaging device (not shown), moves the electrode catheter 14into the patient's heart 16 (as shown in more detail in FIG. 1 a).

When the electrode catheter 14 reaches the patient's heart 16,electrodes 20 at the tip of the electrode catheter 14 may be implementedto electrically map the myocardium 22 (i.e., muscular tissue in theheart wall) and locate a target tissue 24. After locating the targettissue 24, the user must move the electrode catheter 14 into contact andelectrically couple the catheter electrode 14 with the target tissue 24before applying ablative energy to form an ablative lesion or lesions.The electrode-tissue contact refers to the condition when the catheterelectrode 14 physically touches the target tissue 24 thereby causing amechanical coupling between the catheter electrode 14 and the targettissue 24. Electrical coupling refers to the condition when a sufficientportion of electrical energy passes from the catheter electrode 14 tothe target tissue 24 so as to allow efficient lesion creation duringablation. For target tissues with similar electrical and mechanicalproperties, electrical coupling includes mechanical contact. That is,mechanical contact is a subset of electrical coupling. Thus, thecatheter electrode may be substantially electrically coupled with thetarget tissue without being in mechanical contact, but not vice-versa.In other words, if the catheter electrode is in mechanical contact, itis also electrically coupled. The range or sensitivity of electricalcoupling, however, changes for tissues with different electricalproperties. For example, the range of electrical coupling forelectrically conductive myocardial tissue is different from the vesselwalls. Likewise, the range or sensitivity of electrical coupling alsochanges for tissues with different mechanical properties, such as tissuecompliance. For example, the range of electrical coupling for therelatively more compliant smooth atrial wall is different from therelatively less compliant pectinated myocardial tissue. The level ofcontact and electrical coupling are often critical to form sufficientlydeep ablative lesions on the target tissue 24 without damagingsurrounding tissue in the heart 16. The catheter system 10 may beimplemented to measure impedance at the electrode-tissue interface andassess the level of contact (illustrated by display 11) between theelectrode catheter 14 and the target tissue 24, as described in moredetail below.

FIG. 2 a illustrates exemplary levels of electrical contact or couplingbetween an electrode catheter 14 and a target tissue 24. FIG. 2 billustrates exemplary levels of mechanical contact or coupling betweenan electrode catheter 14 and a target tissue 24. Exemplary levels ofcontact or coupling may include “little or no contact” as illustrated bycontact condition 30 a, “light to medium contact” as illustrated bycontact condition 30 b, and “hard contact” as illustrated by contactcondition 30 c. In an exemplary embodiment, the catheter system 10 maybe implemented to display or otherwise output the contact condition forthe user, e.g., as illustrated by light arrays 31 a-c corresponding tocontact conditions 30 a-c, respectively.

Contact condition 30 a (“little or no contact”) may be experiencedbefore the electrode catheter 14 comes into contact with the targettissue 24. Insufficient contact may inhibit or even prevent adequatelesions from being formed when the electrode catheter 14 is operated toapply ablative energy. However, contact condition 30 c (“hard contact”)may result in the formation of lesions which are too deep (e.g., causingperforations in the myocardium 22) and/or the destruction of tissuesurrounding the target tissue 24. Accordingly, the user may desirecontact condition 30 b (“light to medium contact”).

It is noted that the exemplary contact or coupling conditions 30 a-c inFIG. 2 a-b are shown for purposes of illustration and are not intendedto be limiting. Other contact or coupling conditions (e.g., finergranularity between contact conditions) may also exist and/or be desiredby the user. The definition of such contact conditions may depend atleast to some extent on operating conditions, such as, the type oftarget tissue, desired depth of the ablation lesion, and operatingfrequency of the RF radiation, to name only a few examples.

FIG. 3 is a high-level functional block diagram showing the cathetersystem 10 in more detail as it may be implemented to assess contact orcoupling conditions for the electrode catheter 14. It is noted that someof the components typical of conventional tissue ablation systems areshown in simplified form and/or not shown at all in FIG. 1 for purposesof brevity. Such components may nevertheless also be provided as partof, or for use with the catheter system 10. For example, electrodecatheter 14 may include a handle portion, a fluoroscopy imaging device,and/or various other controls, to name only a few examples. Suchcomponents are well understood in the medical devices arts and thereforefurther discussion herein is not necessary for a complete understandingof the invention.

Exemplary catheter system 10 may include a generator 40, such as, e.g.,a radio frequency (RF) generator, and a measurement circuit 42electrically connected to the electrode catheter 14 (as illustrated bywires 44 to the electrode catheter). The electrode catheter 14 may alsobe electrically grounded, e.g., through grounding patch 46 affixed tothe patient's arm or chest (as shown in FIG. 1).

Generator 40 may be operated to emit electrical energy (e.g., RFcurrent) near the tip of the electrode catheter 14. It is noted thatalthough the invention is described herein with reference to RF current,other types of electrical energy may also be used for assessing contactconditions.

In an exemplary embodiment, generator 40 emits a so-called “pinging”(e.g., low) frequency as the electrode catheter 14 approaches the targettissue 24. The “pinging” frequency may be emitted by the same electrodecatheter that is used to apply ablative energy for lesion formation.Alternatively, a separate electrode catheter may be used for applyingthe “pinging” frequency. In such an embodiment, the separate electrodemay be in close contact with (or affixed to) the electrode for applyingablative energy so that a contact or coupling condition can bedetermined for the electrode which will be applying the ablative energy.

The resulting impedance at the electrode-tissue interface may bemeasured during contact or coupling assessment (or “pinging”) using ameasurement circuit 42. In an exemplary embodiment, the measurementcircuit 42 may be a conventionally availableresistance-capacitance-inductance (RCL) meter. Another exemplarymeasurement circuit which may be implemented for determining the phaseangle component is also described in more detail below with reference toFIG. 5. Still other measurement circuits 42 may be implemented and theinvention is not limited to use with any particular type orconfiguration of measurement circuit.

The reactance and/or phase angle component of the impedance measurementsmay be used to determine a contact or coupling condition. The contact orcoupling condition may then be conveyed to the user in real-time forachieving the desired level of contact or coupling for the ablationprocedure. For example, the contact or coupling condition may bedisplayed for the user on a light array (e.g., as illustrated in FIG. 2a-b).

After the user has successfully guided the electrode catheter 14 intothe desired contact or coupling condition with the target tissue 24, agenerator, such as generator 40 or a second generator, may be operatedto generate ablative (e.g., high frequency) energy for forming anablative lesion or lesions on the target tissue 24. In an exemplaryembodiment, the same generator 40 may be used to generate electricalenergy at various frequencies both for the impedance measurements (e.g.,“pinging” frequencies) and for forming the ablative lesion. Inalternative embodiments, however, separate generators or generatingunits may also be implemented without departing from the scope of theinvention.

In an exemplary embodiment, measurement circuit 42 may be operativelyassociated with a processor 50 and memory 52 to analyze the measuredimpedance. By way of example, processor 50 may determine a reactanceand/or phase angle component of the impedance measurement, and based onthe reactance component and/or phase angle, the processor 50 maydetermine a corresponding contact or coupling condition for theelectrode catheter 14. In an exemplary embodiment, contact or couplingconditions corresponding to various reactance and/or phase angles may bepredetermined, e.g., during testing for any of a wide range of tissuetypes and at various frequencies. The contact or coupling conditions maybe stored in memory 52, e.g., as tables or other suitable datastructures. The processor 50 may then access the tables in memory 42 anddetermine a contact or coupling condition corresponding to impedancemeasurement based on the reactance component and/or phase angle. Thecontact or coupling condition may be output for the user, e.g., atdisplay device 54.

It is noted, that the catheter system 10 is not limited to use withprocessor 50 and memory 52. In other embodiments, analog circuitry maybe implemented for assessing contact conditions based on the impedancemeasurement and for outputting a corresponding contact condition. Suchcircuitry may be readily provided by one having ordinary skill in theelectronics arts after having become familiar with the teachings herein,and therefore further discussion is not needed.

It is also noted that display device 54 is not limited to any particulartype of device. For example, display device 54 may be a computer monitorsuch as a liquid-crystal display (LCD). Alternatively, display devicemay be implemented as a light array, wherein one or more light emittingdiodes (LED) are activated in the light array to indicate a contactcondition (e.g., more lights indicating more contact). Indeed, anysuitable output device may be implemented for indicating contactconditions to a user, and is not limited to a display device. Forexample, the contact condition may be output to the user as an audiosignal or tactile feedback (e.g., vibrations) on the handle of theelectrode catheter.

It is further noted that the components of catheter system 10 do notneed to be provided in the same housing. By way of example, measurementcircuit 42 and/or processor 50 and memory 52 may be provided in a handleportion of the electrode catheter 14. In another example, at least partof the measurement circuit 42 may be provided elsewhere in the electrodecatheter 14 (e.g., in the tip portion). In still other examples,processor 50, memory 52, and display device 54 may be provided as aseparate computing device, such as a personal desktop or laptop computerwhich may be operatively associated with other components of thecatheter system 10.

Assessing a contact or coupling condition between the electrode catheter14 and target tissue 24 based on impedance measurements at theelectrode-tissue interface may be better understood with reference toFIGS. 4 and 4 a. FIG. 4 is a model of the electrode catheter 14 incontact with (or coupled to) target tissue 24. The electrode catheter 14is electrically connected to the generator 40 (e.g., an RF generator).In an exemplary embodiment, the circuit may be completed through thetarget tissue 24, showing that current flows through the blood,myocardium, and other organs to the reference electrode, such as agrounding patch 46 on the patient's body (FIG. 1).

As described above, the generator 40 may be operated to generateelectrical energy for emission by the electrode catheter 14. Emissionsare illustrated in FIG. 4 by arrows 60. Also as described above,generator 40 may emit a “pinging” frequency as the electrode catheter 14approaches the target tissue 24 for assessing electrode-tissue contactor coupling. In an exemplary embodiment, this “pinging” frequency may beselected such that inductive, capacitive, and resistive effects otherthan those at the blood-tissue interface do not appreciably affect theimpedance measurements.

In an exemplary application, capacitive effects of the blood and at theelectrode-blood interface (e.g., between the metal electrode catheterand the blood) were found be minimal or even non-existent at frequencieshigher than about 50 kHz. Stray inductance (e.g., due to the relativelythin catheter wires), capacitance and resistance at the electrodeinterface, and capacitance effects of other organs (e.g., the lungs)were also found to be minimal or even non-existent at frequencies higherthan about 50 kHz.

In addition, it was found that resistive effects dominate at theblood-tissue interface for frequencies below 50 kHz because the currentflows into the target tissue 24 primarily via the interstitial fluidspaces 23, and the cell membranes 25 (e.g., bi-lipids or “fat”) act asan insulator. However, at frequencies greater than about 50 kHz, thecell membranes 25 become conductive, and electrical current penetratesthe target tissue 24 through both the interstitial fluid spaces 23 andthe cell membranes 25. Accordingly, the cell membranes act as“capacitors” and the resistive effects are reduced at frequencies aboveabout 50 kHz.

To avoid a risk of creating an ablation lesion during contact orcoupling assessment, it can be desirable to use a low amount of currentand power. A presently preferred range for a current of less than 1 mAis a working frequency in the 50˜500 kHz range.

The frequency choice is mostly based on physiological aspect andengineering aspect and is within the purview of one of ordinary skill inthe art. For physiological aspect, lower frequencies can introducemeasurement errors due to electrode-electrolyte interface. Whenfrequency goes higher to MHz range or above, the parasitic capacitancecan become significant. It is noted, however, that the invention is notlimited to use at any particular frequency or range of frequencies. Thefrequency may depend at least to some extent on operationalconsiderations, such as, e.g., the application, the type of targettissue, and the type of electrical energy being used, to name only a fewexamples.

Assuming, that a desired frequency has been selected for the particularapplication, the model shown in FIG. 4 may be further expressed as asimplified electrical circuit 62, as shown in FIG. 4 a. In the circuit62, generator 40 is represented as an AC source 64. As discussed above,capacitance and resistance at the blood-tissue interface dominateimpedance measurements at low frequency operation such as may be usedfor assessing electrode-tissue contact. Accordingly, other capacitive,inductive, and resistive effects may be ignored and thecapacitive-resistive effects at the blood-tissue interface may berepresented in circuit 62 by a resistor-capacitor (R-C) circuit 66.

The R-C circuit 66 may include a resistor 68 representing the resistiveeffects of blood on impedance, in parallel with a resistor 70 andcapacitor 72 representing the resistive and capacitive effects of thetarget tissue 24 on impedance. When the electrode catheter 14 has no orlittle contact with the target tissue 24, resistive effects of the bloodaffect the R-C circuit 66, and hence also affect the impedancemeasurements. As the electrode catheter 14 is moved into contact withthe target tissue 24, however, the resistive and capacitive effects ofthe target tissue 24 affect the R-C circuit 66, and hence also affectthe impedance measurements.

The effects of resistance and capacitance on impedance measurements maybe better understood with reference to a definition of impedance.Impedance (Z) may be expressed as:Z=R+jX

where:

-   -   R is resistance from the blood and/or tissue;    -   j an imaginary number indicating the term has a phase angle of        +90 degrees; and    -   X is reactance from both capacitance and inductance.

It is observed from the above equation that the magnitude of thereactance component responds to both resistive and capacitive effects ofthe circuit 62. This variation corresponds directly to the level ofcontact or coupling at the electrode-tissue interface, and therefore maybe used to assess the electrode-tissue contact or coupling. By way ofexample, when the electrode catheter 14 is operated at a frequency of100 kHz and is primarily in contact with the blood, the impedance ispurely resistive and the reactance (X) is close to 0 Ohms. When theelectrode catheter 14 contacts the target tissue, the reactancecomponent becomes negative. As the level of contact or coupling isincreased, the reactance component becomes more negative.

Alternatively, contact or coupling conditions may be determined based onthe phase angle. Indeed, determining contact or coupling conditionsbased on the phase angle may be preferred in some applications becausethe phase angle is represented as a trigonometric ratio betweenreactance and resistance. Although the magnitude of the reactancecomponent may be different under varying conditions (e.g., for differentpatients), the phase angle is a relative measurement which tends to beinsensitive to external conditions.

In an exemplary embodiment, the phase angle may be determined from theimpedance measurements (e.g., by the processor 50 in FIG. 3). That is,impedance may be expressed as:Z=|Z|∠φ

where:

-   -   |Z| is the magnitude of the impedance; and    -   φ is the phase angle.

The terms |Z| and φ may further be expressed as:|Z|=√{square root over (R ² +X ²)}; and

${\tan\;\phi} = \frac{X}{R}$

The phase angle also corresponds directly to the level of contact orcoupling at the electrode-tissue interface, and therefore may be used toassess the electrode-tissue contact or coupling. By way of example, whenthe electrode catheter 14 is operated at a frequency of 100 kHz and isprimarily in contact with the blood, the phase angle is close to zero(0). When the electrode catheter 14 contacts the target tissue, thephase angle becomes negative, and the phase angle becomes more negativeas the level of contact or coupling is increased. An example is shown inTable 1 for purposes of illustration.

TABLE 1 Phase Angle Relation to Contact Conditions Phase Angle ContactCondition φ > −3° little or no contact or coupling −3° < φ < −7° mediumcontact or coupling −7° < φ < −10° high contact or coupling φ < −10°excessive contact or coupling

Although impedance measurements may be used to determine the phaseangle, in an alternative embodiment, the measurement circuit 42 may beimplemented as a phase detection circuit to directly determine the phaseangle. An exemplary phase detection circuit 80 is shown in FIG. 5. Phasedetection circuit 80 is shown and described with reference to functionalcomponents. It is noted that a particular hardware configuration is notnecessary for a full understanding of the invention. Implementation ofthe phase detection circuit 80 in digital and/or analog hardware and/orsoftware will be readily apparent to those having ordinary skill in theelectronics art after becoming familiar with the teachings herein.

Exemplary phase detection circuit 80 may include a current sensor 82 andvoltage sensor 84 for measuring current and voltage at theelectrode-tissue interface. The current and voltage measurements may beinput to a phase comparator 86. Phase comparator 86 provides a directcurrent (DC) output voltage proportional to the difference in phasebetween the voltage and current measurements.

In one embodiment, the current sensor 82 may be used to measure theablation current. The sensor can be in series with ablation wire. Forexample, a Coilcraft CST1 current sensing transformer may be placed inseries with the ablation wire. Alternatively, the current wire can passthrough holes of a current sensor, with or without physical connection.In addition, the voltage between the ablation electrode and the groundpatch can be sensed. This voltage can be attenuated so that it can befed into a phase sensing circuit. The phase sensing circuit thenmeasures the current and voltage and determines the phase angle betweenthem, which is then correlated to a coupling level. In this way theablation current can be used to measure the phase angle rather thaninjecting an additional current for the coupling sensing purpose.

Optionally, current measurements may be phase shifted by phase shiftcircuit 88 to facilitate operation of the phase comparator 86 by“correcting” phase lag between the measured current and the measuredvoltage. Also optionally, output from the phase comparator 86 may be“corrected” by phase adjustment circuit 90 to compensate for externalfactors, such as the type of grounding patch 46 being used. A signalscaling circuit 92 may also be provided to amplify the output (e.g.,from milli-volts to volts) for use by various devices (e.g., theprocessor 50 and display device 54 in FIG. 3).

During ablation, the measured impedance, and its component's resistanceand reactance, change with tissue temperature. In such conditions, thechange due to changes in tissue temperature provides a measure of lesionformation during ablation.

It is noted that phase detection circuit 80 shown in FIG. 5 is providedas one example, and is not intended to be limiting. Otherimplementations may also be readily provided by those having ordinaryskill in the electronics arts after becoming familiar with the teachingsherein without departing from the scope of the invention.

Having described exemplary systems for electrode contact assessment,exemplary operational modes may now be better understood with referenceto the block diagrams shown in FIG. 6-8. FIG. 6 is an exemplary blockdiagram 100 showing phase angle measurement for sensing contact orcoupling. FIG. 7 is an exemplary block 200 diagram showing phase anglemeasurement during ablation with both ablation energy and a contactsensing signal applied to the ablation electrode at the same time. FIG.8 is an exemplary block diagram 300 showing phase angle measurementduring ablation with switching between sensing signal and ablationpower. It is noted that 200-series and 300-series reference numbers areused in FIG. 7 and FIG. 8, respectively, to denote similar elements andthese elements may not be described again with reference to FIG. 7 andFIG. 8.

As noted above, the phase angle method of sensing contact or coupling isbased on the fact that (1) tissue is both more resistive and capacitivethan blood, and (2) measured electrode impedance is mostly dependant onthe immediate surrounding materials. Thus, when an electrode moves fromblood to myocardium, the measured impedance value increases and phaseangles change from 0° to negative values (capacitive). Phase angle maybe used to represent the contact or coupling levels because phase angleis a relative term of both resistance and reactance. That is, itprovides a 0° base line when the electrode is in contact with blood, andbecomes increasingly more negative as more contact or coupling isestablished. It also minimizes the influence of the catheter,instrumentation, and physiological variables.

The phase angle measurement may be made by sampling both electricalvoltage (V) 102 and current (I) 104 of a load and calculating the lagbetween those signals as the phase angle. As shown in FIG. 6, a sensingsignal 106 is applied between the ablation electrode 108 and a referenceelectrode 110. This sensing signal 106 can, for example, be between 50to 500 kHz at a small amplitude (<1 mA).

Exemplary instruments may be operated as frequencies of, for example butnot limited to, 100 kHz, 400 kHz and 485 kHz, depending on the referenceelectrode configuration. Both current 104 and voltage 102 are sensed.These two signals are transmitted to a phase comparator 112 to calculatephase angle, which corresponds to the contact or coupling condition ofthe electrode 108. The raw phase angle signal is adjusted in block 114to compensate for external influence on the phase angle, e.g., caused bythe catheter, instrumentation, and physiological variables. It is alsoconditioned for easy interpretation and interface and then output inblock 116 to other equipments for display or further processing.

The phase compensation may be achieved at the beginning of an ablationprocedure. First, the catheter electrode is maneuvered to the middle ofthe heart chamber (e.g., the right atrium or left atrium) so that theelectrode 108 only contacts blood. The system measures the phase angleand uses this value as a baseline for zero contact level. Thisadjustment compensates the fixed phase angles caused by catheter andpatient such as catheter wiring, location of the reference electrode andskin or adiposity if external patch is used.

After the initial zero adjustment, the user may maneuver the catheterelectrode to one or more desired sites to ablate arrhythmic myocardium.In an exemplary embodiment, the phase angle starts to change when theelectrode 108 approaches to say within 3 mm from the myocardium andbecomes increasingly more negative as more contact or coupling isestablished. The user may judge the quality of electrode contact orcoupling before administering the ablation energy based on phase angleoutput. In an exemplary embodiment, this phase angle value is about −3°when a 4 mm ablation electrode actually contacts the myocardium. It isnoted that there are at least two methods to measure phase angle duringablation, as described in more detail now with reference to FIG. 7 andFIG. 8.

In FIG. 7, ablation power 218 is applied to the electrode 208 while thesensing signal 206 is applied as well. The ablation and contact sensingoperate at different frequencies. Accordingly, with filtering, the phaseangle can be measured during ablation without disturbing the ablation ofthe myocardium.

Another option is to switch the phase measurement between the sensingsignal 306 and ablation power 318, as indicated by switch 320 in FIG. 8.When the ablation power 318 is switched off during approach, the smallamplitude sensing signal 306 is switched on and used to measure phaseangle for sensing contact or coupling. When the ablation power 318 isswitched on for the ablation procedure, the voltage and current of thelarge amplitude ablation power 318 are sensed and used as the contact orcoupling indicator during ablation.

FIG. 9 a illustrates one embodiment of an electrode coupling assessmentprotocol 400 (hereafter “assessment protocol 400”) that may be used toassess the coupling of an electrode (e.g., a catheter electrode) withany appropriate tissue, where this assessment is phase angle based.Therefore, the protocol 400 may be used in relation to the embodimentsdiscussed above in relation to FIGS. 6-8. In any case, “coupling” mayinclude an electrical coupling of an electrode with a target tissue, amechanical coupling between an electrode and the target tissue, or both.

Step 402 of the assessment protocol 400 of FIG. 9 a is directed tosending an electrical signal to an electrode. Typically this will beafter the electrode has been positioned at least in the general vicinityof the target tissue (e.g., within a heart chamber, such as the leftatrium). A phase angle is thereafter determined at step 404, and theelectrode coupling is thereafter assessed at step 408 based upon thisphase angle. The electrode coupling assessment from step 408 may becategorized through execution of step 410. However, the categorizationof step 410 may not be required in all instances. In any case, theresult of the assessment from step 408 is output pursuant to step 412.

The electrical signal that is sent pursuant to step 402 of the protocol400 may be at any appropriate frequency. However, only a singlefrequency is required to make the assessment for purposes of theprotocol 400. The phase angle associated with step 404 may be the phaseangle of the impedance. This phase angle may be determined in anyappropriate manner, for instance using a phase sensing circuit of anyappropriate configuration. In one embodiment and using the electricalsignal associated with step 402, the phase angle is determined bymeasuring the current at the electrode, measuring the voltage betweenthe electrode and another electrode (e.g., a return electrode), and thendetermining the phase angle between these current and voltagemeasurements. Another option would be to measure/determine the reactanceand impedance in an appropriate manner, and to then determine the phaseangle from these values (e.g., the sine of the phase angle being theratio of the reactance to the impedance).

The phase angle may be determined using an RCL meter or a phasedetection circuit (e.g., having an oscillator, multiplexer, filter,phase detection circuit), and may be referred to as a phase module. Thisphase module (measurement and/or detection) may be disposed at anyappropriate location, such as by being incorporated into or embedded inthe catheter handle set, by being in the form of a standalone unitbetween the ablation catheter and the power generator, by beingincorporated into or embedded in the power generator, by beingincorporated into an electrophysiology or EP mapping system, or by beingpart of an electrophysiology recording system.

Assessment of the coupling of the electrode with the tissue (step 408 ofthe protocol 400) may be undertaken in any appropriate manner. Forinstance, the phase angle determined through step 404 may be comparedwith one or more benchmark phase angle values (e.g., using a phase anglecomparator). These benchmark phase angle values may be determined/set inany appropriate manner, for instance empirically. These benchmark phaseangle values may be stored in an appropriate data structure, forinstance on a computer-readable data storage medium, or otherwise may bemade available to a phase angle comparator. Generally and in oneembodiment, the phase angle decreases as more electrode-tissue (e.g.,myocardium) coupling exists.

There may be one or more benchmark phase angle values (e.g., a singlebenchmark phase angle value or a range of benchmark phase angle values)for one or more of the following conditions for purposes of thecategorization of step 410 of the assessment protocol 400 of FIG. 9a: 1) insufficient electrode coupling (e.g., an electrode coupling wherethe associated phase angle being less than “A” is equated with aninsufficient electrode coupling); 2) sufficient electrode coupling(e.g., an electrode coupling with an associated phase angle greater than“A” and less than “B” being equated with a sufficient electrodecoupling); and 3) elevated or excessive electrode coupling (e.g., anelectrode coupling where the associated phase angle being greater than“B” is equated with an elevated or excessive electrode coupling). Oneembodiment equates the following phase angle values with the notedconditions:

-   -   insufficient electrode coupling: Φ>−5°    -   sufficient electrode coupling: −5°>Φ>−10°    -   elevated/excessive electrode coupling: Φ<−10°

An “elevated” or “excessive” electrode coupling may beelevated/excessive in relation to the electrical coupling, themechanical coupling, or both (the coupling between the electrode and thetarget tissue). In one embodiment, an elevated/excessive or hardelectrode coupling means an elevated/excessive mechanical contactbetween the electrode and the target tissue. It may be desirable to knowwhen an elevated or excessive mechanical contact exists between theelectrode and tissue for a variety of reasons. For instance, it may bedesirable to avoid an elevated or excessive mechanical contact betweenthe electrode and the target tissue (e.g., to reduce the likelihood ofdirecting the electrode through a tissue wall, membrane, or the like).However, it may also be desirable to know when a sufficient mechanicalforce is being exerted on the target tissue by the electrode (e.g., toincrease the likelihood of directing the electrode through a tissuewall, membrane, or the like to gain access to a desired region on theother side of this tissue wall or membrane).

The result of the assessment of step 408 may be output in anyappropriate manner pursuant to step 412 of the electrode couplingassessment protocol 400 of FIG. 9 a. Any appropriate output may beutilized, for instance visually (e.g., a bar graph or any otherappropriate display at any appropriate location or combination oflocations), audibly (e.g., an alarm), physically (e.g., by vibrating ahandle being held by a physician that is performing an electrode-basedprocedure, and as discussed in more detail herein), or any combinationthereof. A single output may be provided. A combination of two or moreoutputs may also be utilized. One or more outputs may be issued to asingle location or to multiple locations.

FIG. 9 b illustrates one embodiment of an electrode coupling assessmentprotocol 400′ that may be used to assess the coupling of an electrode(e.g., a catheter electrode) with any appropriate tissue, where thisassessment is reactance based. As the protocol 400′ is a variation ofthe protocol 400 of FIG. 9 a, a “single prime” designation is used inrelation the reference numerals that identify the individual steps ofthe protocol 400′ of FIG. 9 b.

Step 402′ of the assessment protocol 400′ of FIG. 9 b is directed tosending an electrical signal. Only a single frequency is required forthe protocol 400′ to provide its assessment. That is, the electrodecoupling assessment may be provided using a single frequency in the caseof the assessment protocol 400′. Typically this will be after theelectrode has been positioned at least in the general vicinity of thetarget tissue (e.g., within a heart chamber). A reactance of theelectrical circuit that includes the electrode and the target tissue isthereafter determined at step 404′. This reactance may be determined inany appropriate manner. For instance, the phase angle may be measured(e.g., in accordance with the foregoing), the impedance may be measured,and the reactance may be calculated from these two values (e.g., thesine of the phase angle is equal to the ratio of the reactance to theimpedance). Another option for determining the reactance would be todetermine the phase or frequency response of a pulse wave.

The electrode coupling is assessed at step 408′ of the protocol 400′based upon the above-noted reactance. This electrode coupling from step408′ may be categorized through execution of step 410′. However, thecategorization of step 410′ may not be required in all instances. In anycase, the result of the assessment is output pursuant to step 412′. Step412′ may correspond with step 412 of the electrode coupling assessmentprotocol 400 of FIG. 9 a.

Assessment of the electrode coupling with the tissue (step 408′ of theprotocol 400′) may be undertaken in any appropriate manner. Forinstance, the reactance determined through step 404′ may be comparedwith one or more benchmark reactance values (e.g., using a reactancecomparator). These benchmark reactance values may be determined/set inany appropriate manner, for instance empirically. These benchmarkreactance values may be stored in an appropriate data structure, forinstance a computer-readable data storage medium, or otherwise may bemade available to a reactance comparator. Generally and in oneembodiment, the reactance decreases as more electrode-tissue (e.g.,myocardium) coupling exists.

There may be one or more benchmark reactance values (e.g., a singlebenchmark reactance value or a range of benchmark reactance values) forone or more of the following conditions for purposes of thecategorization of step 410′: 1) insufficient electrode coupling (e.g.,an electrode coupling where the associated reactance being less than “A”is equated with insufficient electrode coupling); 2) sufficientelectrode coupling (e.g., an electrode coupling with an associatedreactance greater than “A” and less than “B” being equated with asufficient electrode coupling); and 3) elevated or excessive electrodecoupling (e.g., an electrode coupling where the associated reactancebeing greater than “B” is equated with an elevated or excessiveelectrode coupling). One embodiment equates the following reactancevalues for the noted conditions:

-   -   insufficient electrode coupling: X>−5    -   sufficient electrode coupling: −5>X>−15    -   elevated/excessive electrode coupling: X<−15

One benefit of basing the electrode coupling assessment upon phase angleis that the phase angle is more insensitive to changes from patient topatient, or operation setup, than both impedance or reactance whenconsidered alone or individually. Other ways of realizing lesssensitivity to changes from tissue to tissue or such other conditionsmay be utilized to provide an electrode coupling assessment. FIG. 9 cillustrates such an embodiment of an electrode coupling assessmentprotocol 480—a protocol 480 that may be used to assess the coupling ofan electrode (e.g., a catheter electrode) with any appropriate tissue.Step 482 of the assessment protocol 480 is directed to sending anelectrical signal to an electrode at a certain frequency. At least oneelectrical parameter is measured at step 484. What may be characterizedas an “impedance components ratio” is then determined from thismeasurement at step 486. The phrase “impedance components ratio” meansany term that is a ratio of two individual components of the impedance,such as the phase angle (the tangent of the phase angle being equal tothe ratio of reactance to resistance). The impedance components ratiomay be determined in any appropriate manner, such as by simply measuringa phase angle. Other ways for determining the impedance components ratioinclude without limitation determining a resistance and reactance at thefrequency encompassed by step 482, and calculating the impedancecomponents ratio from these two parameters. Using a ratio of twocomponents that relate to impedance may provide less sensitivity tochanges from tissue to tissue for an electrode coupling assessment—anassessment of the coupling between an electrode and the target tissue.

The electrode coupling is assessed at step 488 of the protocol 480. Thiselectrode coupling from step 488 may be categorized through execution ofstep 490, where step 490 may be in accordance with step 410 of theelectrode coupling assessment protocol 400 discussed above in relationto FIG. 9 a. As such, the categorization of step 490 may not be requiredin all instances. In any case, the result of the assessment is outputpursuant to step 492. Step 492 may be in accordance step 412 of theelectrode coupling assessment protocol 400 discussed above in relationto FIG. 9 a.

Each of the protocols of FIGS. 9 a-c encompasses the electrode couplingbeing a mechanical coupling between the electrode and the target tissue(i.e., physical contact), as well as an electrical coupling (e.g., acondition when a sufficient portion of the electrical energy passes fromthe electrode to the target tissue). Any time there is a mechanicalcoupling, there is an electrical coupling. The reverse, however, is nottrue. There may be an electrical coupling without the electrode being incontact with the target tissue. FIG. 10 illustrates a representativeexample of where there is an electrical coupling without havingmechanical contact between an electrode 414 and the target tissue 416.Here, the electrode 414 is disposed within a cavity 418 on the surfaceof the tissue 416, and which provides an electrical coupling between theelectrode 414 and the target tissue 416. Therefore, each of theprotocols of FIGS. 9 a-c may provide an indication of electricalcoupling without requiring mechanical contact between the electrode andthe target tissue.

FIGS. 11 a-c schematically present various configurations that may beused in relation to providing an electrode coupling assessment. Althougheach of these systems will be discussed in relation to an ablationelectrode, this electrode coupling assessment may be used for anyappropriate application where an electrode provides any appropriatefunction or combination of functions. Each of the systems of FIGS. 11a-c may be used to provide the assessment protocols discussed above inrelation to FIGS. 9 a-c. It should also be appreciated that it may bedesirable to utilize various other components to commercially implementthese configurations, such as filters (e.g., as there may be a currentfrom one or more other sources that should be isolated from the currentbeing used to make the coupling assessment), one or more components to“electrically protect” the patient and/or the electrical circuitry usedto make the electrode coupling assessment.

FIG. 11 a illustrates an ablation system 420 that includes an ablationpower source 424, an ablation electrode 422, and a return electrode 426.Any appropriate frequency may be used by the ablation power source 424.Each of the ablation electrode 422 and return electrode 426 may be ofany appropriate size, shape, and/or configuration. Typically theablation electrode 422 will be in the form of a catheter electrode thatis disposed within the patient's body. The return electrode 426 may bedisposed at any appropriate location (e.g., a ground patch disposed onthe skin of a patient; a catheter electrode disposed within the body ofa patient).

Additional components of the ablation system 420 include an electrodecoupling assessment power source 428 (hereafter the “assessment powersource 428”), an assessment return electrode 430, and an electrodecoupling assessment module 432 (hereafter the “assessment module 432”).Any appropriate frequency may be used by the assessment power source428. Typically, the ablation power source 424 will also use asignificantly higher current than the assessment power source 428.

The assessment return electrode 430 may be of any appropriate size,shape, and/or configuration, and may be disposed at any appropriatelocation. One embodiment has the return electrode 426 and the assessmentreturn electrode 430 being in the form of separate structures that aredisposed at different locations. Another embodiment has thefunctionality of the return electrode 426 and the functionality of theassessment return electrode 430 be provided by a single structure (asingle unit that functions as both a return electrode 426 and as anassessment return electrode 430).

The ablation electrode 422 either receives power from the ablation powersource 424 or the assessment power source 428, depending upon theposition of a switch 434 for the ablation system 420. That is, ablationoperations and electrode coupling assessment operations may not besimultaneously conducted in the case of the ablation system 420 of FIG.11 a. During electrode coupling assessment operations, the switch 434 isof course positioned to receive power from the assessment power source428. This allows the assessment module 432 to assess the couplingbetween the ablation electrode 422 and the target tissue. Anyappropriate configuration may be utilized by the assessment module 432to provides its electrode coupling assessment function, includingwithout limitation the various configurations addressed herein (e.g.,assessment based upon phase angle comparisons; assessment based uponreactance comparisons; assessment based upon impedance components ratiocomparisons; assessment based upon identifying the frequency associatedwith a 0° phase frequency or a 0 inductance frequency as will bediscussed below in relation to FIGS. 12 a-b). The assessment module 432may provide the electrode coupling assessment using any of the protocolsof FIGS. 9 a-c from a single frequency.

FIG. 11 b illustrates an ablation system 440 that includes an ablationpower source 444, an ablation electrode 442, and a return electrode 446.Any appropriate frequency may be used by the ablation power source 444.Each of the ablation electrode 442 and return electrode 446 may be ofany appropriate size, shape, and/or configuration. Typically theablation electrode 442 will be in the form of a catheter electrode thatis disposed within the patient's body. The return electrode 446 may bedisposed at any appropriate location (e.g., a ground patch disposed onthe skin of a patient; a catheter electrode disposed within the body ofa patient).

Additional components of the ablation system 440 include an electrodecoupling assessment power source 448 (hereafter the “assessment powersource 448”), an assessment return electrode 450, and an electrodecoupling assessment module 452 (hereafter the “assessment module 452”).Any appropriate frequency may be used by the assessment power source448. However, the ablation power source 444 and the assessment powersource 448 operate at different frequencies in the case of the ablationsystem 440 in order to accommodate the simultaneous execution ofablation and electrode coupling assessment operations. Moreover,typically the ablation power source 444 will also use a significantlyhigher current than the assessment power source 448.

The assessment return electrode 450 may be of any appropriate size,shape, and/or configuration, and may be disposed at any appropriatelocation. One embodiment has the return electrode 446 and the assessmentreturn electrode 450 being in the form of separate structures that aredisposed at different locations. Another embodiment has thefunctionality of the return electrode 446 and the functionality of theassessment return electrode 450 be provided by a single structure (asingle unit that functions as both a return electrode 446 and as anassessment return electrode 450).

The ablation electrode 442 may simultaneously receive power from theablation power source 444 and the assessment power source 448. That is,ablation operations and electrode coupling assessment operations may besimultaneously executed in the case of the ablation system 440 of FIG.11 b. In this regard, the ablation power source 444 and the assessmentpower source 448 again will operate at different frequencies. Theassessment module 452 may provide the electrode coupling assessmentusing any of the protocols of FIGS. 9 a-c from a single frequency. Inany case, the assessment module 452 assesses the coupling between theablation electrode 442 and the target tissue. The discussion presentedabove with regard to the assessment module 432 for the ablation system420 of FIG. 11 a is equally applicable to the assessment module 452 forthe ablation system 440 of FIG. 11 b.

FIG. 11 c illustrates an ablation system 460 that includes an ablationpower source 464, an ablation electrode 462, and a return electrode 466.Any appropriate frequency may be used by the ablation power source 464.Each of the ablation electrode 462 and return electrode 466 may be ofany appropriate size, shape, and/or configuration. Typically theablation electrode 462 will be in the form of a catheter electrode thatis disposed within the patient's body. The return electrode 466 may bedisposed at any appropriate location (e.g., a ground patch disposed onthe skin of a patient; a catheter electrode disposed within the body ofa patient).

Additional components of the ablation system 460 include an electrodecoupling assessment power source 468 (hereafter the “assessment powersource 468”). Any appropriate frequency may be used by the assessmentpower source 468. Typically, the ablation power source 464 will also usea significantly higher current than the assessment power source 468.

The ablation system 460 further includes a pair of electrode couplingassessment modules 472 a, 472 b (hereafter the “assessment module 472 a”and “the assessment module 472 b”). The assessment module 472 a isassociated with the assessment power source 468, while the assessmentmodule 472 b is associated with the ablation power source 464. Bothablation operations and electrode coupling assessment operations utilizethe return electrode 466 in the illustrated embodiment, although it maybe possible to utilize separate return electrodes as in the case of theembodiments of FIGS. 11 a and 11 b discussed above.

The ablation electrode 462 either receives power from the ablation powersource 464 or the assessment power source 468, depending upon theposition of a switch 474 for the ablation system 460. However, electrodecoupling assessment operations may be executed regardless of theposition of the switch 474, unlike the embodiment of FIG. 11 a. When theablation electrode 462 is electrically interconnected with theassessment power source 468 through the switch 474, the assessmentmodule 472 a is used to assess the coupling between the ablationelectrode 462 and the target tissue. When the ablation electrode 462 iselectrically interconnected with the ablation power source 464 throughthe switch 474, the assessment module 472 b is used to assess thecoupling between the ablation electrode 462 and the target tissue. Theassessment modules 427 a, 472 b may each provide an electrode couplingassessment using any of the protocols of FIGS. 9 a-c from a singlefrequency.

Any appropriate configuration may be utilized by each of the assessmentmodule 472 a, 472 b to provide their respective electrode couplingassessment functions, including without limitation the variousconfigurations addressed herein. The discussion presented above withregard to the assessment module 432 for the ablation system 420 of FIG.11 a is equally applicable to the assessment modules 472 a, 472 b forthe ablation system 460 of FIG. 11 c. Typically, the assessment modules472 a, 472 b will be of the same configuration for assessing electrodecoupling, although such may not be required in all instances. When theassessment modules 472 a, 472 b are the same configuration, the ablationpower source 464 and the assessment power source 468 will typicallyoperate at the same frequency. Therefore, the ablation system 460accommodates the assessment of electrode coupling prior to initiatingablation operations (e.g., using an assessment current and theassessment module 472 a), and further accommodates the assessment ofelectrode coupling during ablation operations (e.g., using the actualablation current versus a smaller current, and using the assessmentmodule 472 b). The ablation system 440 of FIG. 11 b also accommodatesthe assessment of electrode coupling during ablation operations, but ituses a separate assessment current versus the actual ablation current.

One of the electrodes used by the assessment module in each of theembodiments of FIGS. 11 a-c is of course the ablation or “active”electrode. Both the electrode coupling assessment module and theablation electrode need another electrode that interfaces with thepatient in some manner to provide their respective functions. FIG. 1 aillustrates one embodiment where the return electrode used by theassessment module and the return electrode that cooperates with theablation electrode to provide electrical energy to the tissue forproviding one or more desired functions are integrated into a commonstructure. More specifically, an ablation electrode 20 (e.g., a catheterelectrode) is disposed in a chamber of the heart 16 (e.g., the leftatrium), and is in the form of a catheter electrode 20. A returnelectrode 20 a (e.g., a catheter electrode) is also disposed in the samechamber of the heart 16 and may be used by each of the assessmentmodules of FIGS. 11 a-c (to assess coupling of the ablation electrode 20with the target tissue 24) and the ablation electrode 20 (to deliverelectrical energy to the target tissue 24 to provide a desired medicalfunction). Therefore, the ablation electrode 20 and the return electrode20 a may be associated with different catheters, and thereby may beindependently moved or manipulated. In one embodiment, the returnelectrode 20 a has a larger surface area than the ablation electrode 20.Each of the ablation electrode 20 and the return electrode 20 a haveelectrode tips that are spaced from each other.

The configuration shown in FIG. 1 a provides two electrodes 20, 20 a ina common heart chamber. Another option would be to have two or moreelectrodes be associated with a common catheter, but where the catheterhas two separated distal portions each with an electrode on a separateelectrode tip on a distal end thereof such that the electrode tips arespaced from each other.

One or more ways of using a phase angle to assess the coupling betweenan active electrode and the target tissue have been presented above.Another way in which a phase angle may be used to assess electrodecoupling is illustrated in FIGS. 12 a-b. FIG. 12 a presents a schematicof an electrode coupling assessment system 500 which includes a variablefrequency source 502, an electrical parameter measurement module 504, anelectrode coupling assessment module 506, and an electrode 508 that isto be coupled with tissue 510 to provide a desired function orcombination of functions (e.g., ablation). The return electrode is notillustrated in FIG. 12 a, but may be of any appropriate type anddisposed at any appropriate location. Generally, the variable frequencysource 502 provides an electrical signal to the electrode 508 forpurposes of transmitting electrical energy to the tissue 510. Theelectrical parameter measurement module 504 may be of any appropriatetype and/or configuration, measures one or more electrical parameters,and provides information used by the electrode coupling assessmentmodule 506. The electrode coupling assessment module 506 assesses thecoupling between the electrode 508 and the tissue 510.

FIG. 12 b presents one embodiment of an electrode coupling protocol 520that may be used by the electrode coupling assessment module 506 of FIG.12 a. One or more electrical signals are sent to the electrode 508through execution of step 524. A baseline coupling condition can beassessed. For example, the baseline coupling condition can be definedpursuant to steps 524-528 of protocol 520. The term “baseline couplingcondition” encompasses a zeroed phase angle or zeroed reactance at adesired frequency in a medium (e.g., blood).

A determination is made through execution of step 525 to determine whenthe electrode is in the desired medium, e.g., the blood. Next, throughthe execution of step 526, the baseline coupling condition isestablished. For example, the physician can activate an input device toindicate the establishment of the baseline coupling condition. Thenprotocol 520 adjusts to the baseline coupling condition in step 528 bycorrecting the phase angle or the reactance to zero.

In an alternative to zeroing the baseline coupling condition, thevalue(s) of the baseline coupling condition established in step 526 maybe stored and used to determine an electrode coupling condition relativeto such a baseline coupling condition. In a second alternative, thebaseline coupling condition may be determined by comparing thedetermined phase angle with one or more predetermined benchmark values.These benchmark values may be determined/set in any appropriate manner,for instance empirically through in vitro, ex vivo, or in vivo studies.These benchmark values may be stored in an appropriate data structure,for instance on a computer-readable data storage medium, or otherwisemay be made available to a phase comparator.

The electrode coupling may be assessed pursuant to step 532 of theprotocol 520 using the baseline coupling condition from step 528. One ormore electrical parameters may be determined in any appropriate mannerand compared with the corresponding value of the baseline couplingcondition from step 528. For instance, the following categories may beprovided: 1) insufficient electrode coupling (e.g., an electrodecoupling where the value(s) associated with a baseline couplingcondition being less than “A” is equated with insufficient electrodecoupling); 2) sufficient electrode coupling (e.g., an electrode couplingwhere the value(s) associated with a baseline coupling condition greaterthan “A” and less than “B” is equated with a sufficient electrodecoupling); and 3) elevated or excessive electrode coupling (e.g., anelectrode coupling where the value(s) associated with a baselinecoupling condition being greater than “B” is equated with an elevated orexcessive electrode coupling).

In another embodiment, the electrical coupling is measured as a functionof a “target frequency”—a frequency that corresponds to a preset valuefor an electrical parameter (e.g., a preset reactance or a phase anglevalue). FIG. 12 c presents one embodiment of an electrode couplingprotocol 620 that may be used by the electrode coupling assessmentmodule 506 of FIG. 12 a. Electrical signals are sent to the electrode508 through execution of step 624. The electrical signals are sent atvarying frequencies. At each frequency sent, step 626 measures thereactance and/or phase. Step 628 compares the measured reactance orphase with a preset value. The frequency at which the reactance or phasematches the preset value is the “target frequency.” Any appropriatevalue may be used for the preset value for purposes of step 628,including a positive value, zero, or a negative value (e.g., a zerophase angle, such that the target frequency may be referred to as a 0°phase frequency; or a zero inductance, such that the target conditionfrequency may be referred to as a 0 inductance frequency).

When the protocol 620 determines that the target frequency exists, theprotocol 620 proceeds to step 630 where the coupling of the electrode508 with the tissue 510 is assessed using the information provided bystep 628, and the result of this assessment is output pursuant to step636 of the protocol 620. Step 636 may be in accordance with step 412 ofthe protocol discussed above in relation to FIG. 9 a.

Assessment of the electrode coupling with the tissue is provided throughstep 630 of the protocol 620 of FIG. 12 c. The target frequency fromstep 628 may be compared with one or more benchmark frequency values(e.g., using a comparator). These benchmark frequency values may bedetermined/set in any appropriate manner. The values can bepredetermined, for instance empirically through in vitro, ex vivo, or invivo studies. These benchmark frequency values may be stored in anappropriate data structure, for instance on a computer-readable datastorage medium. The benchmark frequency values can also be determinedduring the procedure by a physician. For example, a determination can bemade when the electrode is in the desired medium, e.g., the blood. Atthat point the physician can activate an input device to set thebenchmark value for the existing coupling relevant condition.

There may be one or more benchmark frequency values (e.g., a singlebenchmark frequency value or a range of benchmark frequency values) forone or more of the following conditions for purposes of thecategorization for the assessment protocol 620 of FIG. 12 c: 1)insufficient electrode coupling (e.g., an electrode coupling where thetarget frequency being less than “A” is equated with insufficientelectrode coupling); 2) sufficient electrode coupling (e.g., anelectrode coupling where the target frequency is greater than “A” andless than “B” is equated with sufficient electrode coupling); and 3)excessive electrode coupling (e.g., an electrode coupling where thetarget frequency being greater than “B” is equated with an excessiveelectrode coupling). One embodiment equates the following targetfrequency values for the noted conditions (where F_(t) is the targetfrequency for the noted condition):

-   -   insufficient electrode coupling: F_(t)<120 kHz    -   sufficient electrode coupling: 120 kHz<F_(t)<400 kHz    -   elevated/excessive electrode coupling: F_(t)>400 kHz

The protocol 620 of FIG. 12 c may be implemented in any appropriatemanner. For instance, the impedance may be monitored to obtain thetarget phase frequency by sweeping the signal frequency (e.g., inaccordance with the system 500 of FIG. 12 a). This frequency sweep couldbe provided between two appropriate values (e.g., 50 kHz and 1 MHz) andusing any appropriate incremental change between these values for thesweep (e.g., 10-20 kHz increments). This approach uses what may bereferred to as frequency switching, which involves measuring theimpedance one frequency at a time and rotating the frequencies by afrequency synthesizer or the like. Another approach would be to combinemultiple frequencies together, and to determine the impedance at each ofthe individual frequencies from the combined signal through filtering.It should be appreciated that it may be such that interpolation will berequired to determine the frequency associated with the target frequencycondition in some cases (e.g., where the frequency associated with thetarget frequency condition is determined to exist between twofrequencies used by the protocol 620).

Although several embodiments of this invention have been described abovewith a certain degree of particularity, those skilled in the art couldmake numerous alterations to the disclosed embodiments without departingfrom the spirit or scope of this invention. References are only used foridentification purposes to aid the reader's understanding of the presentinvention, and do not create limitations as to the position,orientation, or use of the invention. It is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative only and not limiting. Changes indetail or structure may be made without departing from the spirit of theinvention as defined in the appended claims.

What is claimed:
 1. A medical system comprising an electrical powersource at least interconnectable with a first ablation electrode andconfigured to apply electrical energy to said first ablation electrode;a data structure having a plurality of empirically predeterminedexcessive electrical coupling benchmark values corresponding toempirical amounts of electrical energy passing between a testingelectrode and a corresponding plurality of types of tissue duringablation that form excessively deep lesions in each of the correspondingplurality of types of tissue, wherein each empirically predeterminedexcessive electrical coupling benchmark value is related to electricalconductivity and tissue compliance of each of the correspondingplurality of types of tissue, the corresponding plurality of types oftissue including a target tissue; a coupling assessment moduleconfigured to monitor an electrical coupling condition between saidfirst ablation electrode and the target tissue during application of theelectrical energy to the target tissue, wherein said coupling assessmentmodule is configured, during application of the electrical energy tosaid first ablation electrode, to identify development of an elevatedelectrical coupling condition by comparing at least a portion of animpedance signal measured between said first ablation electrode and thetarget tissue with one of said plurality of empirically predeterminedexcessive electrical coupling benchmark values corresponding to thetarget tissue; and a display device configured to, upon identificationof said development of said elevated electrical coupling condition,generate an output indicative of said elevated electrical couplingcondition.
 2. The medical system of claim 1, wherein said couplingassessment module is configured to utilize a reactance component of saidimpedance signal to identify said development of said elevatedelectrical coupling condition.
 3. The medical system of claim 1, whereinsaid coupling assessment module is configured to compare a phase angleto at least one phase angle benchmark value to identify said elevatedcoupling condition.
 4. The medical system of claim 1, wherein saidcoupling assessment module is configured to compare a frequency, atwhich phase angle has a preset value, to at least one benchmarkfrequency value to identify said elevated coupling condition.
 5. Themedical system of claim 1, wherein said coupling assessment module isconfigured to compare a frequency, at which reactance has a presetvalue, to at least one benchmark frequency value to identify saidelevated coupling condition.
 6. The medical system of claim 1, furthercomprising a return electrode, wherein said first ablation electrode andsaid return electrode are associated with different catheters.
 7. Themedical system of claim 6, wherein each of said first ablation electrodeand said return electrode are disposable in a common heart chamber. 8.The medical system of claim 6, wherein said return electrode has alarger surface area than said first ablation electrode.